In many situations it is desirable to collect semen and other like cryogenically storable tissue (such as blood, bone marrow, or other living tissue suspended in a liquid medium) in a "field" situation where immediate cryogenic storage facilities are not readily available. However, because such tissue dies or loses it effectiveness and efficiency if not frozen for storage in a relatively brief period of time after donation, the donor (whether animal or human) is usually required to be in the same geographic vicinity of the storage facility to facilitate collection and initial freeze-processing of the tissue. Although many advances have been made in the freezing, storage and thawing of such tissue which have met with relatively high success, the art has been hampered by the lack of adequate portable freeze-processing facilities or apparatus which would allow for initial freeze-processing (hereinafter called cryopreservation) and for economic, safe transportation of the same to permanent storage facilities.
Since freezing usually kills living cells, the cells must first be treated with a suitable cryoprotectant chemical, cooled at a specific controlled rate of temperature decrease relative to time, maintained at cryogenic temperature until utilization is desired, and thawed at specific, controlled rate temperature increase. The rate of cooling required to maintain the viability of cells and tissue is dependent, in part, upon the nature of the cryoprotectant chemical, such as glycerol or dimethyl sulfoxide, used and the type of cells or tissue being frozen.
After treatment with the cryoprotectant chemical, the tissue is inserted into elongated plastic tubes or straws, which themselves are of such decreased diameter, size and mass as to expose more surface area of the tissue to the freezing medium and allow relative uniform freezing of the tissue. Although freezing and storage is at low temperatures and must be accomplished at controlled rates, care must be taken not to allow a rate so rapid or so slow as to destroy the tissue and its effectiveness and efficiency. It has been determined by empirical studies that an appropriate rate of freezing can be accomplished by suspending the tissue at predetermined distances (relative to the tissue, cryoprotectant chemical, size or diameter of storage straws, etc.) from the surface of liquid nitrogen with an ambient temperature of 20.degree. C. Temporary storage for transportation purposes has previously been accomplished by use of a Dewar flask or other such container in conjunction with low temperature liquid gases such as liquid nitrogen; however, safety hazards in handling and spillage of the liquid gas or freezing agent during transportation has presented serious drawbacks in such means.
It has been found that controlled rates of freezing can also be accomplished by use of low-temperature solid freezing agent having temperatures of -75.degree. C. or lower, such as frozen PG,3 carbon dioxide, in a similar manner as is used with liquid gas freezing agents. The subject invention is a portable apparatus which utilizes frozen solid freezing agents and allows controlled rate-freezing by varying the amount and quality of insulation material between the tissue and the freezing agents depending parameters such as tissue preserved and the cryoprotectant used.